It has been well known that polyolefins such as crystalline polypropylene are obtained by polymerizing olefins in the presence of so-called Ziegler-Natta catalyst which comprises a compound of a transition metal of Group IV to Group VI in the periodic table and an organometallic compound of a metal of Group I to Group III of the periodic table. Recently, there have been made studies on a process in which crystalline polyolefins of high stereoregularity can be obtained with high polymerization activity using such catalysts as mentioned above.
For example, Japanese Patent Laid-Open Publications No. 209207/1986, No. 104810/1987, No. 104811/1987, No. 104812/1987, No. 104813/1987, No. 311106/1989, No. 318011/1989 and No. 166104/1990 disclose that polyolefins of high stereoregularity can be obtained with high polymerization activity by polymerizing olefins in the presence of a catalyst formed from a titanium-containing sold catalyst component which contains titanium, magnesium, halogen and an electron donor, an organoaluminum compound and an electron donor.
The present applicant has also made a number of proposals with respect to a catalyst for olefin polymerization and an olefin polymerization process by which crystalline polyolefin of high stereoregularity can be obtained with high polymerization activity, as described in, for example, Japanese Patent Laid-Open Publications No. 108385/1975, No. 126590/1975, No. 20297/1976, No. 28189/1976, No. 64586/1976, No. 92885/1976, No. 133625/1976, No. 87489/1977, No. 100596/1977, No. 147688/1977, No. 104593/1977, No. 2580/1978, No. 40093/1978, No. 40094/1978, No. 43094/1978, No. 135102/1980, No. 135103/1980, No. 152710/1980, No. 811/1981, No. 11908/1981, No. 18606/1981, No. 83006/1983, No. 138705/1983, No. 138706/1983, No. 138707/1983, No. 138708/1983, No. 138709/1983, No. 138710/1983, No. 138715/1983, No. 138720/1983, No. 138721/1983, No. 215408/1983, No. 47210/1984, No. 117508/1984, No. 117509/1984, No. 207904/1984, No. 206410/1984, No. 206408/1984, No. 206407/1984, No. 69815/1986, No. 69821/1986, No. 69822/1986, No. 69823/1986, No. 22806/1988, No. 95208/1988, No. 199702/1988, No. 199703/1988, No. 202603/1988, No. 202604/1988, No. 223008/1988, No. 223009/1988, No. 264609/1988, No. 87610/1989, No. 156305/1989, No. 77407/1990, No. 84404/1990, No. 229807/1990, No. 229806/1990 and No. 229805/1990.
By the way, crystalline polypropylene is rigid and usually has a high heat distortion temperature, a high melting point and a high crystallization temperature, and hence it shows excellent properties such as high heat resistance, high crystallization speed and high transparency. Accordingly, crystalline polypropylene has been applied to various uses such as containers and films. Since rigidity and heat resistance of polypropylene are enhanced with increase of crystallinity, polypropylene having high crystallinity can be applied to such uses as require higher rigidity and higher heat resistance. Further, in the conventional uses, a product formed from the polypropylene can be made thin or an amount of a filler to be added can be reduced, that is, weight-saving can be attained.
A propylene block copolymer usually comprises a crystalline polypropylene portion and a non-crystalline polymer portion, and has excellent properties such as lightweight and good balance between rigidity, a heat distortion temperature and impact resistance. Accordingly, the propylene block copolymer has been applied to various uses such as structural materials for containers and electrical appliances and automotive interior trims. Since rigidity and heat resistance of a propylene block copolymer are enhanced with increase of crystallinity of the crystalline polypropylene portion, a propylene block copolymer containing a polypropylene portion of high crystallinity can be applied to such uses as require higher rigidity and higher heat resistance. Further, in the conventional uses, a product formed from the the propylene block copolymer can be made thin or an amount of a filler to be added can be reduced, that is, weight-saving can be attained.
The crystallinity of crystalline polypropylene has been conventionally heightened by a method of adding a nucleating agent or other method, but the conventional crystalline polypropylene has an isotactic pentad value (pentad isotacticity) by the NMR measurement of about 90 to 95%, and the improvement of the rigidity and the heat resistance is limited to a certain extent. Accordingly, there have been keenly desired the advent of a crystalline polypropylene having a prominently high isotactic pentad value, namely a crystalline polypropylene having a high stereoregularity, and the advent of a propylene block copolymer containing a crystalline polypropylene portion having a prominently high isotactic pentad value, namely a propylene block copolymer containing a crystalline polypropylene portion having a high stereoregularity.
Films made of the conventional crystalline polypropylene are not always sufficient in moisture resistance, and hence the advent of a crystalline polypropylene excellent in the moisture resistance as well as in the rigidity and the heat resistance has been also desired.
In the case of molding the above-mentioned crystalline polypropylene, moldability of a resin is improved when a melt viscosity of the resin is low, and hence a resin temperature is generally elevated to lower the melt viscosity of the resin. However, if the resin is molded at a high temperature, the resin tends to be thermally decomposed or deteriorated to sometimes cause various problems such as coloring of the resulting molded product, occurrence of cracks, lowering of long-term heat stability and weathering resistance, and reduction of rigidity and heat resistance.
Further, sheets or films made of the conventional crystalline polypropylene are not always sufficient in the moisture resistance in some uses, and accordingly the advent of a crystalline polypropylene excellent in the moisture resistance as well as in the rigidity and the heat resistance has been desired.
The present inventors have earnestly studied to solve the above-mentioned problems, and as a result, they have found that a propylene polymer composition comprising a propylene polymer (or a propylene block copolymer) which has a much higher stereoregularity than a conventional one and an extremely long mesochain and a specific stabilizer shows high rigidity, high heat resistance and high moisture resistance, and moreover is excellent in heat stability during the molding stage, long-term heat stability of the molded product and weathering resistance thereof as compared with a conventional crystalline polypropylene. Thus, the present invention has been accomplished.